1. Field of the Invention
This invention is directed to an internal media cleaning device for improved treatment of hazardous industrial and/or domestic wastewater, in aerobic fluidized bed reactors. In particular, the invention provides an effective and economical means for separating excess biomass from particulate media, while maintaining the cleaned particulate media within the reactor.
2. Prior Art
Once the particulate media becomes heavily coated with excess biomass in conventional prior art aerobic fluidized bed reactors, the density of the individual media particle decreases and their terminal velocity becomes less than that of the velocity of the influent waste stream. This results in flotation of the particulate media into the effluent stream and their removal from the reactor.
Conventionally, excess biomass growth on the particulate media has been removed by subjecting the individual particles to high levels of turbulence that cause shearing of the biomass from the particulate media. The most common means to achieve biomass removal has been to allow the heavily biomass coated particulate media to float out of the reactor through the effluent line and then create a turbulent zone in a supplemental reaction vessel. Such turbulent zones have been created by passing the particulate media through a pump with rubber impellers or mechanically mixing the particles within the supplemental reactor. The biomass is then typically separated from the particulate media and the particulate media thereafter returned to the reactor.
An alternative conventional design is directed to removing the biomass from the particulate media within the top portion of the reactor by creating a turbulent zone by use of mechanical mixers. See in this regard, Jeris et al., "Biological Fluidized-Bed Treatment for BOD and Nitrogen Removal," Journal WPCF, vol. 49, no. 5, pp. 816-831, 1977.
An additional alternative utilizes intermittent air scouring within the bed to create turbulence throughout the entire bed (Jennings, Paul A., "A Mathematical Model for Biological Activity in Expanded Bed Absorption Columns," PhD Thesis, Univ. of Illinois at Urbana-Champaign, 1975; Greddertz et al., "Offline Regeneration of Granular Activated Carbon,"ASCE Journal of Environmental Engineering, vol. 114, no. 5, pp. 1063-1076, 1988). A variation of this design involves an expansion of the reactor's diameter near its top that results in a reduction of the fluid's upward velocity to create a media settling zone. The particulate media is then cleaned by periodic air scouring within this settling zone (Livingston, A. G., "Biodegradation of 3,4-Dichloraniline in a Fluidized Bed Bioreactor and a Steady-State Biofilm Kinetic Model," Biotechnology and Bioengineering, vol. 38, pp. 260-272, 1991.) These described conventional devices generally prevent the particulate media from spilling into the effluent line during high turbulence cleaning by reducing the bed height by reducing the influent flow rate.
Some United States Patents covering the control of biomass in fluidized bed reactor designs using the methods discussed above or variations of these methods include U.S. Pat. Nos. 4,009,098, 4,009,099, 4,322,296, 4,681,685 and 4,707,252.
Jeris, U.S. Pat. Nos. 4,009,098, 4,009,099, and 4,182,675, disclose fluidized processes that include a means for separating biomass from media bed particles to which they are attached. The relevant disclosures in the three aforementioned Jeris patents are very similar as reflected in the embodiments disclosed in U.S. Pat. No. 4,009,098. One embodiment of U.S. Pat. No. 4,009,098, disclosed at col. 6, lines 24-38, involves removing a predetermined quantity of particles in an external abrasion vessel, and then, abrading the particles either mechanically with a rotating knife or with compressed air or water sprays. A second embodiment of U.S. Pat. No. 4,009,098, disclosed at col. 6, lines 42-50, involves the use of a rotating flexible agitator in the fluidized bed zone for separation and removal of biomass from media particles. Other separating means are disclosed, as follows; "other mechanical mixers, baffle plates and other abrasion-type surfaces or even water or compressed air jets directed upwardly and sidewardly against column walls to create agitation vortices and the like, as well as other suitable conventional agitating means, can be employed within the column" see col. 6, lines 50-56. Furthermore, col. 9, lines 58-65, discloses a means for preventing the agglomeration of bed particles by sub-dividing the fluidized bed into a number of vertical compartments of small cross-sectional area. However, Jeris U.S. Pat. No. 4,009,098, does not show the cleaning apparatus including a screen as set out in the claimed invention.
Durot et al., U.S. Pat. No. 4,707,252, discloses removal of excess biomass from granular material by hydraulic flux combined with a mechanically rotating device for "disembedding" granular material against a fixed cutting grill. However, this cutting grill does not function as a filter; instead, it allows granular material to pass through.
Hickey et al., EP 0,007,783, discloses placement of an internal separator column with a draw-off port in the fluidized bed reactor to separate biomass from particulate media. Shearing forces necessary to effect removal of biomass from particulate media can be generated, as disclosed by separate embodiments, either by a mixing blade, or by sonic or ultrasonic shear. Other embodiments disclose using an external loop with an external pump to effect mechanical shear or an external loop with constricted or sinuous flow path for hydraulic shear. Compressed air, with or without a mixing motor, is another alternate embodiment for an internal separating means. Sheared particulates settle back into the fluidized bed; sheared biomass is removed via a draw-off port.